1. Field of the Invention
This invention relates to oscillator design, and, more particularly, to the design of more accurate low-power oscillators.
2. Description of the Related Art
Wireless protocols like Bluetooth and WLAN (wireless local area network) make provisions for the devices to be placed in sleep mode, but such devices must typically maintain a clock with some amount of accuracy, for example 250 ppm (parts per million) for Bluetooth. Typically, this can be achieved using an external 32 KHz sleep clock, but this adds expense if the crystal doesn't already exist in the system, or if it's hard to route the clock to the device. An internal low power oscillator (LPO) may be designed on-chip, but the oscillation frequency of the LPOs may change as voltage and temperature drift. With careful design, this change may only be a few percent across the entire temperature range, but in terms of ppm 1% may equal 10,000 ppm. In standard CMOS, without a crystal, it is generally difficult to achieve the 250 ppm goal with a single oscillator.
Oscillators are typically electronic circuits that convert energy from direct-current sources into periodically varying electrical signals, or voltages. That is, an oscillator typically operates by utilizing the electrical behavior of its circuit elements to convert a steady state input signal into a periodic, time variant output signal. In some implementations the signal produced by an oscillator may be sinusoidal in appearance, such as a sine wave, in other implementations it may appear as a square wave, triangular wave, or a variety of other repeatable signals. As mentioned above, many of today's integrated circuits that require oscillators, such as timer circuits, need to include the oscillators on-chip in order to meet cost and area requirements. The behavior of such on-chip oscillators is typically affected by the technology used to fabricate the integrated circuit, temperature changes, and supply voltage changes. For example, many widely used fabrication processes today are based on complementary metal-oxide-semiconductor (CMOS) technology, where each specific qualified CMOS process varies slightly from another, and parts manufactured within a given specific qualified CMOS process also vary with respect to each other, within certain tolerances.
One common type of oscillator is the relaxation oscillator. Typically a relaxation oscillator achieves its oscillating output by charging a capacitor to some event or switching threshold. The event discharges the capacitor, and its recharge time determines the repetition time of the events or switching. Similarly, an oscillating output could also be achieved by discharging instead of charging the capacitor to reach the event or switching threshold. Typically the capacitor is charged through a resistor, with the values of the resistor and the capacitor, which define an RC time constant, determining the rate, or frequency, of the oscillation. For example, decreasing the value of the resistor may increase the oscillation frequency, and increasing the value of the resistor may decrease the oscillation frequency. Typical relaxation oscillators whose frequency is determined by an RC time constant may be prone to temperature and voltage supply variations, since the resistor(s) and capacitor(s) (corresponding to the RC time constant) are temperature dependent, as well as supply dependent with the amplitude of the signals typically affecting the oscillator frequency.
Since the accuracy of the oscillator may determine the proper functionality of the integrated circuit or system in which the oscillator is configured, it is important to ensure that the frequency (or period) of oscillation does not fall outside certain required limits with temperature and/or supply voltage variations. Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.